I use a tape recorder tape head de-magnetizer on the plungers and the coil stops. If there is still a problem, then I cut a small circle of self-stick mylar (just like the mylar which protects playfields) and stick it to the end of the plunger. This creates a barrier to the magnetism so the plunger can't stick to the stop. The mylar lasts a very long time. I have done this for 30 years without problems. This is very common on Bally linear flipper assemblies.